CN100343924C - Method for measuring the relative extent of burnout of combustion elements in a pebble-bed high-temperature reactor (HTR) and a corresponding device - Google Patents

Abstract

The methods used to date to measure burnout of combustion elements in pebble bed high temperature reactors typically have an error of 10% especially at high cycle rates. The measuring method of the present invention is very fast on the one hand, and very accurate on the other hand, the error is 1-2%. The method advantageously comprises: a) the combustion element is removed from the reactor and transported to the measurement location; b) the combustion element is subjected to a flux of thermal neutrons; c) the first detector measures gamma rays emitted by the combustion element; d) when the When the predetermined first threshold value is reached, the combustion element is sent back to the reactor, and when it is lower than the first threshold value, the combustion element continues to perform steps e) to f); e) the second detector measures the high energy above 1 MeV emitted by the combustion element gamma rays; f) when a predetermined second threshold value is exceeded, the combustion element is returned to the reactor, and when the second threshold value is lower than the second threshold value, the combustion element is removed from the combustion element cycle. Therefore, the device of the present invention has a first detector for measuring all gamma rays emitted from the combustion element and a second detector for measuring high-energy gamma rays of 1 MeV or more emitted from the combustion element.

Description

Method and device for measuring relative value of burning loss of combustion element

The invention relates to a measurement method of a combustion element, in particular to a measurement method which can be used to determine the burning loss of a pebble bed high temperature reactor (HTR).

When operating a built-in pebble bed HTR (such as an AVR or THTR) with multiple outlets, a certain proportion of circulating combustion elements (BE) must be removed from the circuit in order to make room for new combustion elements to be introduced. In view of the good fuel economy of the nuclear system, however, consideration should be given as far as possible to the removal of the combustion elements with the greatest burnout. For this purpose, each individual cycle combustion element is tested. What is measured here is a physical quantity representing the degree of burnout. The homogeneity of the burn-in values is not essential here for measurement accuracy, but a large measurement effect and good reproducibility of the measured values are important. This value is used to determine whether the combustion elements are fed back into the reactor core, if necessary to which core region, or whether they are removed.

The fission process takes place inside the reactor core, in which fission products are produced from nuclear fuel in combustion elements. If the circulating individual combustion elements pass from the reactor core into the spherical outlet tube, the fission process is then stopped. The fission products in the combustion element are radioactive and emit gamma rays. For the different combustion elements, the total gamma rays irradiated by the combustion elements and measured are correlated with their burnout under otherwise identical conditions, eg after exiting the combustion elements from the reactor core after the same time.

To date, different measurement methods for determining the degree of burnout of spherical combustion elements have been used.

Gamma spectroscopic measurements of Cs ¹³⁷ in the combustion elements can be performed with semiconductor detectors cooled by liquid nitrogen for an AVR (Professional Working Group Experimental Reactor) cycle rate of about 500 per day based on relatively small combustion elements. The measurement is inexpensive, providing a measurement accuracy in the range of ±2% at high burnout BE (burning element) within an acceptable measurement time of 20 to 40 seconds.

For a modern modular pebble bed nuclear power reactor - such as Siemens' HTR module or South Africa's PBMR - the cycle rate is much higher (about 4000 combustion elements per day) relative to the AVR and the decay time of the combustion elements in the spherical outlet tube is relatively short ( about two days), so that the direct transfer of the AVR measurement method cannot be transferred to such reactors solely on the basis of the small measurement time available. A shorter measurement time necessarily leads to a larger measurement error. Of course of more environmental interest, the analysis of the Cs ¹³⁷ line is obviously imprecise due to the relatively small decay time of the combustion element. The high radioactivity of short-lived fission products has a strong influence on the gamma measurement of Cs ¹³⁷ , since the analytically typical 662 keV line of Cs ¹³⁷ is clearly influenced by the adjacent line (Nachbarlinien). This involves on the one hand the strong 658 keV line of Nb ⁹⁷ (effective half-life 16.8 hours), the weak 661 keV line of Ba ¹⁴⁰ (half-life 12.8 days) and the strong 668 keV line of I ¹³² (effective half-life 76.3 hours). A corresponding correction of the measured signal of Cs ¹³⁷ therefore generally requires complex measuring techniques. The rapid circulation in combination with a short bulb and the short residence time in the bulb thus leads to a strong adverse effect on the reproducibility of the Cs measurement. For this reason, there is no concrete experience with real reactors. The assessment of the achievable accuracy is very similar to those skilled in the art. According to the general opinion, the average error for the combustion element with high burning loss is not less than ±10%.

Therefore, a corresponding expert group recommends simply measuring the total gamma activity of the combustion elements selectively for modern modular pebble bed nuclear power reactors.

The gamma activity of the radioactive combustion elements is predominantly in the reactor core, but short-lived fission products, when the decay time is not too long, predominate after the combustion elements have exited the core. The contribution of long-lived fission products to radiation intensity is practically negligible. A burnt element with a lower degree of burnout produces a higher power than a burnt element with a high burnout in the reactor core and therefore also before it exits the core, and thus has a higher (short-lived) gamma activity. The measurement effect, ie the gamma radiation of the low burn-out combustion elements and the high burn-out combustion elements, differs significantly. (The gamma activity of low-loss burner elements for AVRs, shown by their longer decay times of one month on average, is always 3 to 4 times higher than with high-loss burner elements). Although not very precise, the method is simple and fast (measurement time about one second).

The combination of total gamma activity and radiometric measurements of Cs ¹³⁷ is considered prior art. A simple gamma measurement (for example 1 second) is performed here for all combustion elements. Only when a high-burning combustion element is identified, with a gamma emission value below a previously defined limit value, is the measurement of eg parallel-extending Cs ¹³⁷ delayed (approximately 10 seconds). Targeting of the combustion element - re-introduction or removal - is only possible when analyzing the measurement of Cs ¹³⁷ .

In the case of this combined method, which regularly allows longer measurement times for the Cs measurement, greater averaging errors are taken into account for high-loss combustion elements. Those skilled in the art consider an accuracy of ±4% to ±20% to be sufficient.

The object of the present invention is to provide a method for measuring spherical combustion elements by means of which the degree of burnout of the combustion elements can be determined for short decay times of the combustion elements and in short measurement times during cyclic operation of a pebble bed reactor.

Furthermore, it is the object of the invention to provide a corresponding device for carrying out the measuring method described above.

The object of the invention is achieved by a method for determining burnout of spherical combustion elements and a device for carrying out the method.

The invention does not describe a method for determining the absolute value of the burnout (for example in % FIMA = fission rate of initial metal atoms) of a spherical combustion element. Nor is the invention intended to measure the burn loss of low burn loss combustion elements. This burner element was identified by a simple gamma measurement due to its significantly higher gamma activity.

The new method according to the invention provides, in particular, that for combustion elements classified as higher burnout by means of a simple gamma measurement, the degree of burnout is further specified, in particular taking into account the possibility of removal.

The subject of the invention is a method for measuring the degree of burnout of spherical combustion elements, similar to the method described previously as a combined method. Brief, simple gamma measurements are made from the combustion elements removed from the reactor core. Via a first predetermined limit value for the gamma activity, the measured combustion elements are thus classified into low-loss or high-loss combustion elements. Combustion elements identified as having a higher burnout are subjected to another measurement. This second measurement is based on the assumption that the more fissions occur in the combustion element when excited with thermal neutrons, the smaller the burn losses. During the fission process, intense gamma rays are emitted spontaneously. The intensity of the intense gamma rays, especially in the energy region above 2 MeV, can therefore likewise be regarded as a measure of burnout of the combustion element.

The method for measurement was performed as follows. The burner element spheres are removed from the reactor core, for example, in the context of circulation and passed to the measuring point. There, the burner element ball is subjected to a flux of thermal neutrons which leads to nuclear fission in the burner element. In addition to the existing gamma activity of the fission products, nuclear fission processes also result in the emission of so-called spontaneous emission lines and in the form of intense gamma rays. The intense gamma rays contain on average more energy than the fission product gamma rays.

In a first measuring step, the total gamma activity of the combustion element is measured with a first detector. This measurement is usually fast (about 1 second), but not very precise. This measurement is only used to make a first assessment of the burnout of the tested combustion element. For a given reactor, the frequency of combustion elements with a defined total gamma activity occurs according to a statistical frequency distribution. In this case, it also depends on at which point in time the combustion element is measured after removal from the reactor core. High burn-out combustion elements also have only few fission products, so that the activity of gamma rays emitted by these products is very small. If an upper limit for the gamma rays is specified, above which the combustion elements measured are always returned to the reactor core, a preselection of the combustion elements is possible, wherein a further measurement is still awaited. The limit value can determine the frequency distribution accordingly. For example, a limit value is determined such that a maximum of 20% of all measured combustion elements have a measured activity below the limit value. Only at this 20% is the second measurement, which is advantageously carried out in parallel, awaited.

In the second measuring step of the method according to the invention, only the intense gamma radiation of the combustion element is detected with a corresponding second detector. The method according to the invention advantageously uses an existing reactor as a neutron source to generate nuclear fission in the combustion element. Detectors suitable for this must in particular be able to detect radiation of high energy, preferably above 2 Mev. For this energy selected measurement eg a NaI scintillation counter is sufficient. The second detector should also at least be able to handle gamma total pulse rates of at least more than 107/s, in particular more than 108/s.

The gamma activity of fission products is usually significantly greater than that of fission due to the short decay time of the fission products within the combustion element. In order that the useful signal of the intense gamma rays is not superimposed too much by the gamma activity of the fission products of the less energetic combustion elements, several solutions can be implemented individually or in combination.

1. It is advantageous to use a detector for the second measuring step which can handle very high pulse rates, i.e. has very good time resolution and therefore has only a small error.

2. Furthermore, the more energetic (intense ) and the ratio of gamma rays with little energy is not good for intense rays. Such shielding can be produced, for example, by lead filters.

3. In order to carry out the second measurement particularly precisely, the second detector should advantageously be arranged such that its optimum operating range lies exactly at the radiation value emitted by the (higher burn-out) combustion element concerned. This also has the disadvantage that the radiation values of the low-burn-out combustion element are significantly higher than the optimum operating range of the second detector. To avoid possible damage to the second detector, various measures can be taken. In particular, another suitable shielding can be used so that the second detector is not overloaded when measuring low-burn-off combustion elements. Or switch off the second detector when measuring low-burn-out combustion elements, which is particularly easy to implement for measurements 1 and 2 carried out in succession.

4. The number of induced nuclear fissions in the combustion element increases with increasing neutron flux (measured flux). In order to achieve a thermal neutron flux which is as high as possible at the measuring point of the combustion element, a real reactor is therefore suitable in particular as a neutron source. In principle also other neutron sources are suitable.

5. The measuring location is advantageously surrounded by water. As a result, the reactivity of the subcritical measuring device is increased, and as many neutrons released during the fission as possible are used for further fission. The combustion elements to be measured themselves influence the reactivity of the device with their fissile substances. This leads to an enhancement of the measurement effect.

6. In order to increase the accuracy of the second measurement, a plurality of second detectors can optionally be provided, which add the determined counting results in parallel.

7. Furthermore, it is provided that a plurality of combustion elements are measured simultaneously and in parallel at a plurality of measurement points. Without changing the number of cycle times, each measurement is available for several measurement times, which regularly and positively affects the measurement accuracy.

8. In principle, it is also advantageously possible to extend the time between removal of the combustion element from the reactor core and its measurement (intermediate time), since the gamma rays of the fission products decrease over time according to their decay, but the gamma rays of the induced fission are not affected. Its disadvantage is causing expensive structural changes or unfavorable reactor operation.

The method according to the invention allows an accurate representation (with an error of only about 1-2%) simply by the degree of burnout of the combustion element. The method is therefore particularly suitable for carrying out the decision whether the combustion elements circulating in a high temperature reactor (HTR) should be removed from the reactor cycle or returned to the reactor core. The method here favorably supports this decision as follows:

a) the combustion element is removed from the reactor and transported to the measurement location,

b) the combustion element is subjected to thermal neutron flow,

c) the first detector measures gamma rays emitted by the combustion element,

d) when a predetermined first threshold value is exceeded, the combustion elements are fed directly to the reactor again, and when this threshold value is below, the combustion elements steps e to f are then carried out,

e) The second detector measures high-energy gamma rays above 1 MeV emitted by the combustion element,

f) The combustion elements are fed back into the reactor when a predetermined second limit value is exceeded, and the combustion elements are removed from the combustion element cycle when this limit value is undershot.

The subject matter of the present invention will be described in detail below with reference to the embodiments and drawings, without thereby limiting the subject matter of the present invention.

The drawing shows a horizontal section through an exemplary embodiment of a device for carrying out the method according to the invention. in,

1 Reactor, the outside of the biological shell

2 Thermal column (graphite) with thermal neutron flow

3 ball catheter

4 water tanks

5 biological shielding

6 Combustion element at measuring position

7 Replacement plugs for detectors

8 A second energy-selective gamma detector with high temporal resolution

9 Connecting cables for pulse processing

10 Detector shield and energy filter, e.g. made of lead

11 The first gamma detector

12 For circulators with fixings for combustion elements at measuring points

Components (schematically shown)

The method according to the invention is carried out in a device as follows:

The combustion element 6 to be measured, removed from the reactor core, is brought into a defined measuring position 12 in which it is subjected to the thermal neutron flux 2 . Nuclear fission occurs in the combustion element in connection with burn-out or fissile material still contained in the combustion element 6, the strength of which is determined by testing techniques. The measured values here are intense high-energy gamma rays, which are emitted by the fission products produced during the measurement following the fission process (spontaneous emission). It is used here that the energy of the intense gamma rays is on average higher than the gamma rays emitted by the fission products in the combustion elements. The energy-selected gamma measuring instrument thus detects intensely more energetic gamma rays. A suitable detection system is, for example, a high-resolution scintillation counter 8 with a high temporal resolution, for which the energy resolution is sufficient.

A small, higher energy fraction of the gamma rays of the fission products in the combustion element (which falls into the range of the intense gamma rays to be measured) can be measured together without a major influence on the accuracy of the measurement, because the total of the combustion element to be measured The gamma rays are likewise associated with burning, that is to say in the same way. The higher the burn loss, the smaller the fissile material content, the smaller the fission activity when measured, the smaller the intense gamma rays, and the smaller the total combustion element radioactivity. (The scheme for simply measuring the total gamma rays of the combustion element as a measurement method has been mentioned above.)

Two features that have been implemented so far characterize the principle of the new method. The main difficulty with this method is that, due to the short decay time (typically 2 days) of BE, its gamma activity is very high (interfering signal), compared to which the intense gamma rays which are useful signals are completely in the background. The following additional features of the method are also important in order to achieve the above-mentioned desired accuracy, ie to accumulate a statistically large enough number of useful signals within a short measuring time.

A gamma measuring instrument 8 (second detector) which handles very high pulse rates can be used, thus obtaining very high temporal resolution. The shielding 10 between the measuring device 8 and the combustion element 6 to be measured is designed such that the measuring device is already operating within its maximum possible range for combustion elements with higher burnout, ie combustion elements emitting weaker radiation. Burning elements that emit more intense radiation and have not yet burned out sufficiently can no longer be detected by the second detector 8 . This makes sense for the combination method described by means of a simple gamma measurement with the first detector 11 .

The necessary shielding 10 between the combustion element 6 and the measuring instrument 8 (second detector) is implemented in the form of lead in order to achieve the greatest possible energy filter effect (preferably permeable to intense gamma rays).

Owing to the short measurement time, the consumption of fissile material for the measurement is completely negligible even at very high neutron fluxes (measurement fluxes). In order to achieve a good accuracy of the method, it is therefore possible to work with as high a measurement flow as possible. So no external neutron source is used, but the reactor core itself is advantageously used as neutron supplier. For this purpose, the reactor is similar to a research reactor, such as the Dido of the Jülich GmbH Research Center, where the "thermal column" 2, that is, the radially extending tube between the side reactor and the outside of the biological hood, is as far as possible only disconnected from the reactor vessel. , The continuous graphite connection is kept as high as possible in the middle of the reactor core. There is a measurement location 12 immediately in front of the outer end face of the graphite. Furthermore, the measuring location 12 is advantageously surrounded by water 4 . This increases the reactivity of the subcritical measuring device and maximizes the use of the neutrons released during the fission for further fission. The combustion element to be measured itself influences the reactivity of the device with its fissile content. This leads to a strengthening of the measurement effect.

With regard to the practical implementation, the measuring point 12 is advantageously arranged in a component of the spherical charging device 3 in which the ball is always present. For this purpose, in particular a ball delivery preselector is provided, by means of which the desired target of the measuring ball (on the ball bed or out of the ball) is controlled. Arranging the delivery preselector at the middle height of the reactor core (before the "hot column") also has the advantage of a long high delivery from the area of the lower end of the extraction pipe of the balls to the upper reversing point of the delivery pipe leading to the pebble bed The path is divided into two subsections, so that a single pneumatic ball transport process uses a low delivery pressure and a sufficient delivery volume.

The biological shield 5 surrounding the measuring device and the further gamma detector 11 belong to the entire measuring device. The detector is arranged to operate at a high count rate at the measuring point 12 for combustion elements with low burn-out (for example, after a core flow). The gamma activity of all combustion elements (and other balls) removed from the measuring point is measured by means of the detector 11 for the combination method mentioned above. If the measurement result of the detector 11 is greater than a defined limit value, the measured combustion element has not burned sufficiently and is returned to the reactor core without waiting for a second measurement. Below this limit value, the measurement of the combustion element also waits for the detector 8, after which the ball target is first determined (removed or returned). This is done again by comparing the measurement result with another limit value. Remove the combustion element when the limit value is undershot.

These two limit values can be derived from the frequency distribution of a large number (for example 300) of measurement results of previously measured combustion elements. This quantity is equal to the area under the distribution curve. For determining the limit value, a value is sought on the measured value scale which divides the distribution area with a predetermined value relationship. If, for example, it is provided that 20% of all measured combustion elements are also to be measured with the second detector element 8 , then the distribution area of the measurement results of the first detector 11 is divided in a ratio of 2:8. 20% of all measurements are below the first limit value. Furthermore, if it is assumed that the reactor is operated at a ratio of 1:10, i.e. every fresh burner element added is cycled and therefore - always the long-term average - 1 burner element must be taken out of 10 cycled burner elements, the share taken 10%, a value is sought in the frequency distribution of the measurement results of the second detector 8 which divides the distribution area into two halves of equal size. Combustion elements whose measurement results fall below this second limit value are removed. The share withdrawn is 10%, so that a 1:10 operating mode is required. The frequency distribution and thus the limit value calculation can be added up after each combustion element measurement. When the reactor power is changed the measurement is multiplied by the ratio of the new relative to the previous power before its processing.

If the second detector 10 is damaged by a supersaturation of gamma rays in the open state, advantageously the two measurements are not started in parallel, but first the gamma rays are measured only with the first detector 11 . The operating voltage of the second detector 8 is only switched on, for example, if the measured result falls below the first limit value.

It can be seen that the measuring device does not always have to be arranged in front of the biological cover 1 as shown, but can also be arranged in a recess of the biological cover. As a result, the "hot column" is shortened and the measurement flow 2 is made larger. Measurements can even be taken directly on the outside of the reactor pressure vessel. The second detector 8 is of course strongly exposed to the gamma rays of the reactor core. A high measurement flow 2 is significant for the accuracy of the method, in which case a constant gamma background can be tolerated for measurement as long as it is not dominant.

Claims (19)

1. A method for measuring a relative value of the burnout of a combustion element, comprising the steps of: a) a combustion element (6) is removed from the reactor and transported to a measurement location (12), b) the combustion element (6) is subjected to a flow of neutrons (2), thereby producing nuclear fission in the combustion element, c) a first detector (11) measures all gamma rays emitted by the combustion element, d) when a predetermined first limit value is exceeded, the combustion elements are fed back to the reactor, and when the first limit value is undershot, steps e) to f) are carried out for the combustion elements, e) a second detector (8) measures high-energy gamma rays above 1 MeV emitted by the combustion element, f) When a predetermined second limit value is exceeded, the combustion elements are fed back to the reactor, and when the second limit value is undershot, the combustion elements are withdrawn from the combustion element cycle. 2. The method according to claim 1, wherein the second detector (8) measures high-energy gamma rays above 2 MeV emitted from the combustion element. 3. Method according to claim 1 or 2, wherein a second detector (8) is used having a count rate of at least ¹⁰⁷ /sec. 4. A method according to claim 1 or 2, wherein a scintillation counter is used as the second detector (8). 5. The method according to claim 1 or 2, wherein a shield is provided between the measuring location (12) and the second detector (8). 6. The method according to claim 5, wherein a lead filter is provided as the shielding between the measuring location (12) and the second detector (8). 7. The method according to claim 1 or 2, wherein the first detector (11) measures the gamma rays of the combustion element (6) in less than 2 seconds. 8. The method according to claim 1 or 2, wherein the second detector (8) measures the gamma rays of the combustion element (6) in less than 30 seconds. 9. The method according to claim 1 or 2, wherein the combustion element (6) is surrounded by water at the measurement location. 10. The method as claimed in claim 1 or 2, wherein a first limit value for the first gamma measurement is determined such that a fraction of the combustion elements corresponding to the reactor operating mode is below the first limit value. 11. A method according to claim 1 or 2, wherein a first threshold value for the first gamma measurement is determined such that a maximum of 20% of the combustion elements for a 1:10 reactor mode of operation are below the first threshold values, where a 1:10 operating mode of the reactor means that every fresh burner element added is cycled and therefore 1 burner element must be removed from 10 cycled burner elements. 12. The method as claimed in claim 1 or 2, wherein a second limit value for the second measurement is determined such that a fraction of all measured combustion elements that is appropriate for the reactor operating mode falls below the second limit value. 13. A method according to claim 1 or 2, wherein a second threshold value for the second measurement is determined such that for a 1:10 reactor mode of operation at most 15% of all measured combustion elements are below the second limit value The limit value, where the operating mode of the reactor of 1:10 means that every fresh burner element added is cycled and therefore 1 burner element must be removed out of 10 cycled burner elements. 14. Apparatus for carrying out the method according to any one of claims 1 to 13, having: a) a neutron source which produces a thermal neutron flux (2), b) for fixing a measuring position (12) of a combustion element to be measured so that the combustion element is subjected to the thermal neutron flow, c) a first detector (11) capable of measuring all gamma rays emitted by the combustion element (6), d) A second detector (8) capable of measuring high-energy gamma rays above 1 MeV emitted by the combustion element. 15. The device according to claim 14, wherein the device has a shield (10) between the measuring location (12) and the second detector (8). 16. The device according to claim 15, wherein the device has a lead filter as shield (10). 17. The device according to claim 14, wherein the device has a scintillation counter as the second detector (8). 18. Apparatus according to claim 14, wherein the second detector (8) has a count rate of at least ¹⁰⁷ /sec. 19. The device according to claim 14, wherein the measuring location (12) is at least partially surrounded by water.

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